Ionizing radiation, like other toxic hazards, has detrimental health effects, including cancer, at high doses and high dose rates.  There is however, considerable controversy regarding the health effects of low-level radiation, at or below typical natural background levels.

Current "risk per exposure" ratios have been derived from a study of the health of atomic bomb survivors at Hiroshima and Nagasaki.  These survivors received tens to hundreds of rem instantaneous (acute) exposure from the initial explosions, plus long-term (chronic) exposure from fallout.

Various regulatory bodies, including the U.S. Environmetal Protection Agency (EPA) have extrapolated these risk ratios, based on high doses, down to much lower dose levels, and all the way down to zero dose.  This theoretical extrapolation is called the linear-no-threshold (LNT) hypothesis.  In effect, it postulates that even the smallest incremental dose of radiation has an associated small risk associated with it.

Proponents of the theoretical LNT model argue that, since we do not know much about health effects at very low doses, it is prudent and conservative to presume that they exist, and that the LNT model is simple and represents a reasonable upper bound for the risks.

Opponents of the theoretical LNT argue that it is not supported by any scientific evidence of health effects at very low doses, and that regulations based on the LNT are excessively costly and do not achieve any real measurable public health benefit.

The material below discusses health risks from low levels of radiation and risk assessment methodology.

Health Risks from Exposure to Low Levels of Ionizing Radiation

EPA Radiation Risk Assessment Guidance

Interstate Technology & Regulatory Council (ITRC)